1. Field of the Invention
The present invention relates to a hydraulic control system for a steplessly variable power transmission.
2. Description of the Prior Art
There has been known a steplessly variable power transmission provided with a belt-pulley mechanism having a primary pulley device connected with an engine and a secondary pulley device connected with vehicle wheels wherein an engine power is transmitted from the primary pulley device to the secondary pulley device with a predetermined pulley speed ratio therebetween through a belt member. Effective diameters of the primary and secondary pulley devices are controlled by a hydraulic mechanism for providing the predetermined pulley speed ratio.
Japanese Patent Public Disclosure No. 58-88252, laid open to the public on May 26, 1983, discloses a control method of a line pressure of the hydraulic mechanism of the steplessly variable power transmission wherein the line pressure of the hydraulic mechanism is determined in accordance with an engine output torque and the pulley speed ratio to improve a transmission efficiency and a durability of the belt member. The engine torque is determined in accordance with an engine operating condition in light of an engine output property memorized in a controller constituted by a micro computer.
In the conventional steplessly variable power transmission, the engine torque is transmitted to the belt-pulley mechanism through a torque converter disposed between the belt-pulley mechanism and the engine. As a result, a magnitude of input torque of the torque converter or the engine output torque does not always correspond to the output torque of the torque converter or an input torque of the belt-pulley mechanism. In detail, the torque converter functions to amplify the torque therethrough in a torque amplifying zone of a pump turbine speed ratio between a pump device connected with an engine output shaft and a turbine device connected with an output shaft of the torque converter. Therefore, the engine output torque is different from the output torque of the torque converter. It will be understood that there may be produced a slippage between the belt member and the pulley device because the line pressure of the hydraulic mechanism for controlling the diameter of the belt-pulley mechanism is controlled in accordance with the engine output torque or input torque of the torque converter rather than an input torque of the belt-pulley mechanism or output torque of the torque converter.
Where the line pressure is increased to prevent the belt-pulley mechanism from slipping, the line pressure may be maintained at an unduly high value in a fluid coupling zone in which no torque amplification occurs in the torque converter, unlike the torque amplifying zone, so that a head loss of a hydraulic pump and a friction loss between the belt member and pulley device are increased.